1. Field of the Invention
This invention relates to a pneumatic radial tire for large-size vehicles such as truck, bus, construction vehicle and the like, and more particularly to a pneumatic radial tire for large-size vehicles having an excellent durability of a bead portion.
2. Description of Related Art
A pneumatic radial tire to be mounted onto a large-size vehicle, for example, a construction vehicle such as a large-size dump truck used in a large-size engineering work site or ore mining site is subjected to severer service conditions on rough roads under a heavy load, so that there are frequently caused troubles in a bead portion contacting with a rim.
In this type of the tire, a protection rubber layer called as a rubber chafer is arranged in a portion contacting with a rim. As performances required in the protection rubber layer, there are mentioned (1) air permeability is less, (2) a bead portion hardly slips against the rim, and (3) cracks hardly occur through friction with a rim flange. In order to satisfy the above performances (1) and (2), it is advantageous to use rubber having high modulus and a less deterioration with a lapse of time in the protection rubber layer. In order to satisfy the above performance (3), it is advantageous to use rubber having a low modulus, so that both rubbers are conflicting with each other in the material properties.
When the protection rubber layer having a high modulus is used for satisfying the performances (1) and (2), therefore, there has hitherto been adopted a structure of disposing a wire chafer on an outside of a carcass as a countermeasure for preventing the occurrence of cracks due to the friction with the rim flange. In this case, however, separation failure is easy to be caused in an end of the wire chafer, particularly an end thereof located at the outside of the carcass as a starting point, so that this countermeasure becomes disadvantageous in view of the durability.
On the other hand, when the protection rubber layer having a low modulus is used for satisfying the performance (3), the performances (1) and (2) are ensured by approaching a position of a bead core to a side of the rim to increase a compression ratio of a bead base rubber after the rim assembling onto the rim to thereby strongly push a bead portion toward the rim. In this case, however, the rim assembling operation is obstructed as the compression ratio of the bead base rubber becomes higher and also the falling down of a sidewall portion increases due to the low modulus of the protection rubber layer, so that it is disadvantageous to have a bad influence upon the steering stability.